"We may not be the only ones in the world who have credibility, but I do think we have credibility for being objective and independent," he said.

Mr Blix, who is due to retire from his post in June, briefed the UN security council on his readiness to send inspection teams back to Iraq.

Earlier, in a BBC radio interview, he said the coalition had appeared to use "shaky" evidence, including forged documents, as a pretext for making war on Iraq.

Afterwards he said it was "conspicuous" that coalition forces had so far failed to find "anything relevant" in their search for proscribed weapons.

The White House, which accused Mr Blix of hindering its drive for international support for the war, is reluctant to see him return to Iraq and has already sent its own teams to search for illegal weapons. It is is recruiting former UN inspectors from the US, Britain and Australia to verify any discoveries.

"For the time being, and for the foreseeable future, we see it as a coalition activity," the US ambassador to the UN, John Negroponte, said after the security council meeting.

The US desire to sideline the UN, and the information on which the US and Britain based their case for military action, also attracted sharp criticism from the former foreign secretary Robin Cook.

Yesterday Mr Cook said he doubted that there was a single figure in the intelligence services who believed that a weapon of mass destruction in working order would be found in Iraq, and he called for Mr Blix to be allowed back to Iraq "on the next plane".

Avoiding a direct attack on Tony Blair, he accused the White House of "reinventing the term weapon of mass destruction to cover any artillery shell with a chemical content, or any biological toxin, even if it had not been fitted to a weapon".

Mr Blix said that given the concern for safety in Iraq it was too early to send inspectors back, but the decision could not be postponed indefinitely.

Speaking in Vienna, the UN secretary general, Kofi Annan, backed Mr Blix, insisting that his security council mandate remained valid, despite his team's withdrawal from Iraq before the war began.

"As soon as the situation settled, the expectation was that they could go back and continue their work, and verify that Iraq has been rid of weapons of mass destruction," he added.

Earlier in the day Mr Blix used a BBC radio interview to criticise the Bush administration for its use of questionable intelligence, including forged documents, in its effort to show that Iraq possessed banned weapons. He said it was "very, very disturbing" that US intelligence had failed to identify as fake documents suggesting that Iraq had tried to buy uranium from Niger.

British officials now admit that documents purporting to show that Iraq was trying to obtain uranium to develop nuclear weapons were forgeries.

The claim that that Saddam Hussein was trying to procure uranium from Niger, in west Africa, was presented as hard intelligence-based evidence in the dossier on Iraq's possession of weapons of mass destruction which the government published in September.

The claim was taken seriously by the UN weapons inspectors until, with the help of independent experts, they found that the documents were forged.

In early March Mohamed El Baradei, director of the International Atomic Energy Agency and chief nuclear inspector for Iraq, contradicted the Downing Street and MI6 claim.

He said the documents used to substantiate them were "not authentic".

Though intelligence officials say there was additional evidence pointing to Iraq's attempt to procure uranium, Whitehall now agrees that the intelligence documents it sent to the IAEA - the UN's nuclear watchdog - and the US were fabricated.

Mr Blix also criticised the Americans for attempting to undermine his authority.

"They felt that stories about these things would be useful to have and they let it out," he said.